<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>第一次作业</title>
</head>
<body>

<p>"Hello World!"</p>
<button onclick="myFunction()">变为红色</button>
<button onclick="myFunction1()">复原</button>
<script>
function myFunction() {
    document.querySelector("p").style.backgroundColor = "red";
}
function myFunction1() {
    document.querySelector("p").style.backgroundColor = "";
}
</script>

</body>
</html>